To get the grid to align on Roll20, create a new map page that is 32" wide by 40" high at 5 ft/ inch. Upload this map onto it. Drag the upper left corner of the map to the upper left corner of the white grid, it should snap into place. Find the middle edge of the bottom side and drag it down the bottom of the grid, then grab the middle of the right edge and drag it to the right edge of the grid. The map should now cover all of the white grid surface. right click on the map and change the layer to "Map", the grid should now show over the map. The tiles of the walkway are 2.5ft squares so a 2x2 group of tiles should be 5ft x 5 ft and fill one map grid.